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Thread: Schaub Lorenz Power Board

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    Default Schaub Lorenz Power Board

    Hi Guys,

    I'm hoping someone will be able to help here.

    I've got a Schaub Lorenz tv....Model VMD 3700 PHF.

    The bloke who gave it to me said it wasn't working. I didn't bother powering it up to check, just took the back off.
    I changed 5 caps (2 swollen, 3 shrunken PVC) circled red, and mains fuse which was a T5amp.



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    When I switched it on, the fuse immediately exploded. Originally a ceramic fuse but I only had glass to replace it.
    Another look at the board revealed 2 resistors burnt and shattered out one side, circled red.
    I don't know if I burnt the resistors or they were like it when I got.



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    Does anyone know what the value of these 2 resistors (R801, R802) are supposed to be ?

    and What other bad components could there be ?


    Loopyloo
    Last edited by loopyloo; 06-01-12 at 02:40 PM.



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    check the transisters on the heat sink next to R802 for s/c and if ok check D803 s/c bridge rec but if resisters gon then will be the switching transisters i would think the R801 R802 are safty resisters for the transisters about .5 to 1 see if conected to b+ and other to ground
    Last edited by CHRISS; 06-01-12 at 11:25 PM.

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    Quote Originally Posted by loopyloo View Post
    When I switched it on, the fuse immediately exploded.
    What other bad components could there be ?


    Loopyloo
    Only short circuit silicon can produce such violent fuse explosion. Check every semiconductor on the primary side. Secondary side caps don't let the smoke out of primary side components.

    Cheers
    "The saddest aspect of life right now is that science gathers knowledge faster than society gathers wisdom." - Issac Asimov

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    Quote Originally Posted by TVguy View Post
    Only short circuit silicon can produce such violent fuse explosion. Check every semiconductor on the primary side. Secondary side caps don't let the smoke out of primary side components.

    Cheers
    concur...
    When I explained to the guy what avatar I wanted, that wasn't what I meant!

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    I pulled one of the resistors and found what looks a bit silly like 0.011 but too hard to read.
    I'll get into checking the rest of the stuff tomorrow and then come back here.

    I'm gonna guess it's that piece of silicon right above the large cap that's taken the fuse out. It's gotta be close to the mains input I reckon.

    Till tomorrow then !
    Last edited by loopyloo; 06-01-12 at 11:59 PM.

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    hard to see but that may be a FET next to main filter cap but just Check all of them for s/c as tvguy said,
    you will find a short, will have to disconect from pcb as you go till clear short and find witch part is short
    in the old days we use to replace all silicon in primary side and any feedback from secondary side

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    Don't mean to hijack but sounds like a 80cm LCD i have here so when the result is found please post it.

    SS Dave

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    Quote Originally Posted by SS Dave View Post
    Don't mean to hijack but sounds like a 80cm LCD i have here so when the result is found please post it.

    SS Dave
    This one's a 37", and don't worry, I will post results.
    Last edited by loopyloo; 07-01-12 at 07:51 AM.

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    If you refer to the second photo.
    The black lolly above the cap is a W20NM60. This fet is short on all pins.
    The 2 to the right are both 07N60C3. Both short G-D but the bottom one is also blown out the back, but not through to the heatsink as all 3 fets have a silicon insulator behind them.

    The top resistor is 0.11 ohm and the bottom 0.22ohm. I think they are 2 watt (unsure). Both burnt.

    The W20NM60 is a TO-247 and I think it's a STW20NM60FD but I can't find where to buy it.

    The two 07N60C3 are just TO-220 style but I can't seem to find a part number to order them from anywhere either.

    More help required.

    Thanks guys,
    Loopy
    Last edited by loopyloo; 07-01-12 at 10:46 PM.

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    I just check the bridge rectifier and found it to be short too.
    It's a KBJ606G. Another component to cause me trouble as I can't find it anywhere to buy either.
    All the ones I found that are 600v 6A are built back to front. This one's got the + on the left.

    Has anyone got all these parts listed ?

    Loopy

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    Element14 has the STW20NM60 in stock, part number 1752206, $8.30 each.

    They also have SPP07N60C3 , part number 1056546, $3.18 each.

    The bridge rectifier is available from several overseas suppliers but shipping costs could make it unattractive.
    All the ones I found that are 600v 6A are built back to front. This one's got the + on the left.
    Couldnt you just turn it around?
    Last edited by Bibliophile; 09-01-12 at 10:09 AM.
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    Default Looking for parts?

    I have found this web site very usefull:



    Just type in the manufacturers part number or device number, even if you've only got a partial, and there's a good chance you will find what you want but not necessarily where you want it. Works for other components besides semiconductors.
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    Quote Originally Posted by Bibliophile View Post
    Element14 has the STW20NM60 in stock, part number 1752206, $8.30 each.

    They also have SPP07N60C3 , part number 1056546, $3.18 each.

    The bridge rectifier is available from several overseas suppliers but shipping costs could make it unattractive.

    Couldnt you just turn it around?
    I don't know how you found those numbers, I couldn't...But I really appreciate your help here.

    As for turning it around...I'm not sure. Heatsinking will be on the back-plane and it would need to be flat anyway. This one's stepped.
    There's no room on the board to turn the aluminium heatsink around.

    Loopy

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    I've looked at several that would meet the electrical requirements and are also flat on both sides so reversing them would not be a problem. Lead pitch is also a consideration and none that I looked at had 7.5 +/-0.2 mm spacing.
    Last edited by Bibliophile; 09-01-12 at 12:22 PM. Reason: Wrong information.
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    Quote Originally Posted by Bibliophile View Post
    I've looked at several that would meet the electrical requirements and are also flat on both sides so reversing them would not be a problem. What is difficult is the lead pitch of 7.5 +/- 0.2mm. I couldn't find any like that available locally.
    That findchips site is excellent, wish I knew about it earlier.
    I've got Element 14 on the job as we speak. They suggested 4084845 but it's pin spacing is about 5mm. Not sure if it will stretch.
    I guess I could lower it on the heatsink with a bit of fiddling.

    Loopy
    Last edited by loopyloo; 09-01-12 at 12:32 PM.

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    Perhaps you could mount it upside down and run wire links to the board.
    Be sure to sleeve them with heatshrink or something to avoid having mains voltage exposed.
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    Quote Originally Posted by Bibliophile View Post
    Perhaps you could mount it upside down and run wire links to the board.
    Be sure to sleeve them with heatshrink or something to avoid having mains voltage exposed.
    Good call,

    Many times I have flown leads out of odd bridge recifiers. Alternatively using robust discrete diodes has worked for me. Nice to see component level repair attempted.

    Cheers
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    Turn it upside down eh ! ?
    Normally I'm pretty damn good at thinking outside the square but you caught me on this one.
    It's a great idea so if it's tricky to fit I'll just do that instead.

    Well I've ordered the parts now and just have to wait. Couple of them are 6-7 days.

    Thanks guys, I'll see ya back here in a week.

    Loopy

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    The rest of the parts came today so I was able to put it back together.

    As soon as i applied the mains, there was a pop.
    Then the blue lamp on the front just flashed slowly and continuously.

    Pulled the power board out again and found the W20NM60 and the 0.11 resistor gone again. Looks like the resistor flashes over to the heatsink with burn marks visible. The other parts I replaced are still ok.

    I have checked everything I can think of. Got me stumped now.

    Loopy

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    Quote Originally Posted by loopyloo View Post

    As soon as i applied the mains, there was a pop.
    Then the blue lamp on the front just flashed slowly and continuously.

    Pulled the power board out again and found the W20NM60 and the 0.11 resistor gone again. Looks like the resistor flashes over to the heatsink with burn marks visible. The other parts I replaced are still ok.

    I have checked everything I can think of. Got me stumped now.

    Loopy
    Very bold to apply full mains to check a SMPS. A variac, isolation transformer and oscilloscope are your best friends when attempting this type of repair. There should be a control IC on the foil side of the PCB. From the images I conclude that all components on the left are for active power factor correction, the small eight pin IC is for standby power and the large transformer on the RHS is to supply AC for rectification to supply 24 volts etc.

    Regards
    Last edited by TVguy; 18-01-12 at 07:43 PM.
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